<button action="<?php echo $this->getSubmitUrl($_product) ?>" method="post" id="product_addtocart_form"<?php if($_product->getOptions()): ?> enctype="multipart/form-data"<?php endif; ?> type="button" class="button btn-cart" onclick="productAddToCartForm.submit(this)"><span><span>Comprar</span></span></button>

var productAddToCartForm = new VarienForm('product_addtocart_form');
    productAddToCartForm.submit = function(button, url) {
        alert("Entrou na função");
        if (this.validator.validate()) {
            var form = this.form;
            var oldUrl = form.action;

            if (url) {
               form.action = url;
            }
            var e = null;
        //Start of our new ajax code
            if(!url){
            url = jQuery('#product_addtocart_form').attr('action');
            }
            alert("Redirecionamento");
            //url = url.replace("onestepcheckout/index","ajax/index/");
            var data = jQuery('#product_addtocart_form').serialize();
            data += '&isAjax=1';
            try {
                alert("Try");
                jQuery.ajax({
                      url: url,
                      dataType: 'json',
                      type : 'post',
                      data: data,
                      success: function(data){
                            alert(data.status + ": " + data.message);
                            if(jQuery('.block-cart')){
                                jQuery('.block-cart').replaceWith(data.sidebar);
                            }
                            if(jQuery('.header .links')){
                                jQuery('.header .links').replaceWith(data.toplink);
                                window.location.href = '<?php echo Mage::getUrl("onestepcheckout/index");?>';
                            }
                      }
                });
            } catch (e) {
            }

            this.form.action = oldUrl;
            if (e) {
                throw e;
            }

            if (button && button != 'undefined') {
                button.disabled = true;
            }
        }
        alert("Saindo...");
    }.bind(productAddToCartForm);

    productAddToCartForm.submitLight = function(button, url){
        alert("Entrou no submitLight");
        if(this.validator) {
            var nv = Validation.methods;
            delete Validation.methods['required-entry'];
            delete Validation.methods['validate-one-required'];
            delete Validation.methods['validate-one-required-by-name'];
            if (this.validator.validate()) {
                if (url) {
                    this.form.action = url;
                }
                this.form.submit();
            }
            Object.extend(Validation.methods, nv);
        }
    }.bind(productAddToCartForm);